An MRI apparatus is an imaging apparatus configured to excite nuclear spin of an object placed in a static magnetic field with a radio frequency (RE) pulse having the Larmor frequency and reconstruct an image based on magnetic resonance (MR) signals emitted from the object due to the excitation.
In recent years, technology to further enhance static-magnetic-field intensity of an MRI apparatus has been developed. A 3 Tesla type MRI apparatus whose static-magnetic-field intensity is two times stronger than a conventional 1.5 Tesla type MRI apparatus has already begun to spread. Clinically practical use of an ultrahigh magnetic-field type MRI apparatus in which static-magnetic-field intensity is further enhanced (e.g., 7 Tesla type and 11 Tesla type) is anticipated in the future.
A signal to noise ratio (SNR) of an MR signal is enhanced by enhancing static-magnetic-field intensity. It is also said that an SNR is enhanced substantially in proportion to static-magnetic-field intensity. Spatial resolution and time resolution can be improved by enhancing an SNR.
However, stronger static-magnetic-field intensity leads to longer longitudinal relaxation time T1. Thus, for instance, when plural excitation pulses are used for imaging under a fast spin echo (FSE) type pulse sequence, it is required to wait recovery of longitudinal magnetization by lengthening a repetition time TR. As a result, an imaging time is lengthened.
Meanwhile, a specific absorption rate (SAR) is known as an index indicative of magnitude of thermal influence on a human body. An SAR is in proportion to square of product of static-magnetic-field intensity B0 and an flip angle FA of an RF pulse (i.e., an SAR is in proportion to (B0*FA)2). Thus, in an MRI apparatus with higher static-magnetic-field intensity, requirements for reducing the SAR become stronger. In particular, in an FSE-type pulse sequence, in which refocusing pulses of high flip angles (e.g., 180°) are sequentially applied subsequent to a 90° pulse, the requirements for reducing the SAR become much stronger.